Radiotherapeutic apparatus is well-known, and consists of a source of radiation which emits a beam of radiation that is directed toward a patient in order to destroy or otherwise harm tumourous cells within the patient. Usually, the beam is collimated in order to limit its spatial extent to a desired region within the patient, usually the tumour or a sub-section of the tumour. The source can be a linear accelerator for high-energy (MV) x-radiation, or an isotopic source such as Co-60.
The source is often rotated around the patient in order to irradiate the desired region from a number of different directions, thereby reducing the dose applied to healthy tissue around the desired region. The shape of the defined desired region can change dynamically as the source rotates, in order to build up a complex dose distribution for tumours with more challenging shapes and/or which are located near to sensitive areas.
As the dose distribution becomes more closely tied to the exact shape of the tumour, and as the accuracy of the dose delivery improves, it has become necessary to know the current position of the patient, their internal organs, and the tumour with greater accuracy. As a result, low-energy x-ray sources are often provided on the apparatus in addition to the high-energy therapeutic source, to allow for x-ray or CT imaging of the patient before or during treatment. Portal imagers are often provided, which detect the therapeutic beam after attenuation by the patient.
Various configurations exist for the radiation source. One common arrangement is for the source to be mounted on the end of an arm that extends away from a large upright cylindrical gantry that is rotatable around its horizontal axis. The arm is located off-centre with respect to the gantry, and the source directs the beam of radiation towards the axis. In this way, as the gantry rotates, the source irradiates the point at which the beam and the rotation axis meet (the “isocentre”) from all directions.
Another is shown in US2013/0158382A1, in which a source is located within a rotateable section of an annular ring disposed around the patient. The fixed part of the ring is mounted at its base onto a support so as to allow it to be tilted away from a vertical orientation; thus, as the source rotates it irradiates the isocentre from all angles, whereas tilting the ring allows the plane in which the various irradiation directions lie to be varied. However, tilting the ring moves the centre of the ring, thus moving the isocentre and meaning that either all treatment during a particular treatment fraction needs to be at a single tilt angle, or the position of the patient needs to be adjusted during a treatment fraction.
Our previous publication WO2005/041774A1 discloses a radiotherapy apparatus in which the source is mounted on a rotateable ring, via a rotateable union that allows the source to rotate around an axis that is perpendicular to the rotation axis of the ring, which emits a beam collimated towards the intersection of the two rotation axes. Thus, as the ring rotates, the source irradiates a single point along a direction lying on the surface of a cone whose enclosed angle is determined by the angle between the source and the ring.